Muslim immigration, a possible Grexit and Ukraine are all shaking the idea of a borderless Europe.

Europe today is a continent of borders. The second-smallest continent in the world has more than 50 distinct, sovereign nation-states. Many of these are part of the European Union. At the core of the EU project is an effort to reduce the power and significance of these borders without actually abolishing them — in theory, an achievable goal. But history is not kind to theoretical solutions.

Today, Europe faces three converging crises that are ultimately about national borders, what they mean and who controls them. These crises appear distinct: Immigration from the Islamic world, the Greek economic predicament, and the conflict in Ukraine would seem to have little to do with each other. But in fact they all derive, in different ways, from the question of what borders mean.

Europe's borders have been the foundation of both its political morality and its historical catastrophes. The European Enlightenment argued against multinational monarchies and for sovereign nation-states, which were understood to be the territories in which nations existed. Nations came to be defined as groupings of humans who shared a common history, language, set of values and religion — in short, a common culture into which they were born. These groups had the right of national self-determination, the authority to determine their style of government and the people who governed. Above all, these nations lived in a place, and that place had clear boundaries.

The right of national self-determination has created many distinct nations in Europe. And, as nations do, they sometimes distrust and fear one other, which occasionally leads to wars. They also have memories of betrayals and victimizations that stretch back for centuries before the nations became states. Some viewed the borders as unjust, because they placed their compatriots under foreign rule, or as insufficient to national need. The right of self-determination led inevitably to borders, and the question of borders inevitably led to disputes among states. Between 1914 and 1945, Europeans waged a series of wars about national boundaries and about who has the right to live where. This led to one of the greatest slaughters of human history.

The memory of that carnage led to the creation of the European Union. Its founding principle was that this kind of massacre should never happen again. But the union lacked the power to abolish the nation-state — it was too fundamental to the Europeans' sense of identity. And if the nation-state survived, so did the idea of place and borders.

If the nation-state could not be abolished, however, then at least the borders could lose their significance. Thus two principles emerged after World War II: The first, predating the European Union, was that the existing borders of Europe could not be changed. The hope was that by freezing Europe's borders, Europe could abolish war. The second principle, which came with the mature European Union, was that the bloc's internal borders both existed and did not exist. Borders were to define the boundaries of nation-states and preserved the doctrine of national self-determination, but they were not to exist insofar as the movement of goods, of labor and of capital were concerned. This was not absolute — some states were limited in some of these areas — but it was a general principle and goal. This principle is now under attack in three different ways......................